In the literature one often sees the Lorentz invariant parameter \(\chi \)  as defined by [1]}
\(\chi = \frac{e\hbar \sqrt{|u\cdot F \cdot F \cdot u|}}{m^2c^3} = \left. \frac{\mathcal {E}}{\mathcal {E}_\text{S}}\right|_{CF} = a_0 a_q \\= 5.9\times 10^{-2} E[\text{GeV}] \sqrt{I[10^{20} \text{W/cm}^2]}\,,\) 
